Final answer:
Derek is crying due to not having fully developed object permanence, which makes him believe his mom no longer exists when she leaves the room. This developmental stage usually occurs in young children around Derek's age.
Step-by-step explanation:
Derek is likely crying because at one year old, he may not have fully developed the concept known as object permanence. This is a key developmental stage in a child's growth. According to psychologist Jean Piaget, object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, or sensed in any way.
Before developing this concept, children might believe that once something is out of sight, it no longer exists. So, when Derek sees his mom leave a room, he might be upset because he does not realize she still exists outside of his immediate perception.
This may cause him distress, leading to his crying. Answer D, 'Choices A and B', is the most suitable answer for why Derek is crying, as both reasons may be contributing to his reaction.